1. noun, The wing or foil at the rear of a car providing downforce, often made with form rather than function in mind, to adorn the rear of the cars driven around your town centre by mouth-breathers who think they're in The Fast & the Furious.

2. noun, A piece of information likely to ruin dramatic tension in any piece of entertainment if discovered too early. Usually any plot development revealed more than a third of the way through the story. She's a man, he's a ghost, it's his sledge, he's a clone, etc.

3. verb, To ruin any piece of entertainment by revealing such information.

4. verb, To protect others from entertainment being ruined by obscuring such information from them.

1. Look at that mean spoiler on his Honda, it goes well with the over-sized exhaust pipe.

Internet slang: A unknown fact within a series (generally of a future event) that ruins the tension and mystery for newer fans.

What constitutes a spoiler is often debatable but a good rule for determining one is the "3 of 6 rule". The "six" are Who, What, Where, When, Why, and How. If a comment has atleast three of these six then it can generally be called a spoiler.
One of the only exceptions to the "3 of 6 rule" is "X dies" or "X is dead" as this is still considered a spoiler because the When or How is generally considered irrelevent when it comes to deaths.

"Vader, Luke" >This statement is WHO+WHO and thus doesnt convey a spoiler as no information is being revieled.

"Vader is Luke's Father" >This is WHO+WHO+WHAT, and thus fits the "3 of 6 rule" and is a spoiler.

Spoilers are experts from the real boom or story, that you may not know of yet. This is to tell you that if you have not read the book or story but plan to, but don't want to ruin it, to not read that fanfiction. The parts of the novel you may not know yet are written in the text. See fanfiction for more information on those.

"Hey Sarah, this now HP fic has spoilers in it for the new book. Should I read it?"

my team was 8-5 just like the Bolts, and we were fighting for the last spot on the playoffs, and on the last week of the season, the 2-11 Rangers team beat us and became the spoiler of our playoff hopes